Network storage system, management method therefor, and control program product therefor

ABSTRACT

A storage system, a storage management method, and a control program product are provided. The storage system is improved in comfortability, convenience, and economy by reducing the amount of copies in a storage device in a network storage system, by heightening storage efficiency and increasing an access speed. In a network storage system in which a plurality of client terminals are directly connected to a storage device via a network, the storage device includes an MV logical disk that stores read-only shared data and a BV logical disk from/onto which data specific to each client terminal is read/written. A control unit that controls read/write operations includes, with an access management table, an LDK management table which has a reference logical disk number column used to issue a command to refer to the MV logical disk when data other than the write data is read.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2007-062097, filed on Mar. 12, 2007, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a storage system and a managementmethod therefor, and, more particularly, to a network storage system, astorage-device management method, and a control program.

2. Description of the Related Art

Recently, the importance of security enhancement, such as prevention ofprivate information leakage, has risen, and a network boot system hasbeen proposed in which all information is managed and operated on aserver without providing a PC (personal computer) of a client with astorage device, such as a hard disk, and the client PC is booted frominformation on the server. FIG. 6 illustrates one example of the systemformed as described above. Clients 10 are connected to a server 90 via anetwork 20. The server 90 can form a boot image of a client PC on theserver from a boot image stored in a storage device 40 and information,such as TempA, unique to each client, can transmit the resulting bootimage, and can boot the client PC.

Recently, an iSCSI (Internet Small Computer System Interface) protocolhas appeared according to which a client PC and a storage can bedirectly connected together via a network without being connectedthrough a server. In the iSCSI protocol, a remote storage connected bythe Internet can be logically shown in the same way as a local storageusing an SCSI protocol by putting an SCSI command or data on a TCP/IPpacket. The use of the iSCSI protocol makes it possible to remove theabove-mentioned server and to simplify a system structure.

FIG. 7 illustrates a structure in which a client 10 and a storage 40 aredirectly connected together via a network 20 by use of the iSCSIprotocol to eliminate a server. The storage 40 includes a host interface50 that performs read/write processing from the client 10, a cachememory 60 that temporarily stores the data of a logical disk (LDK) 70,the logical disk (LDK) 70 logically formed on one or more physicaldisks, and a control unit 80 that controls these elements. In thestructure formed as described above, in which the client 10 and thestorage 40 are directly connected together, the logical disk (LDK) 70 isallocated to each client 10, and hence each LDK 70 is required to beshown to each client by means of an access management table 81 providedin the control unit 80. A method for showing each LDK to each client bymeans of the access management table 81 is known by, for example,Japanese Published Unexamined Patent Application No. 2000-181636 (NEC)(Patent Document 1). FIG. 7 shows an example in which an LDK1 isallocated to client A, an LDK2 is allocated to client B, and an LDK3 isallocated to client C.

The structure mentioned above has the following problems.

A first problem is that much time is consumed to prepare an LDK used forbooting. The LDK70 provided in the storage 40 is required to be made foreach client 10, and is made according to the following procedure. First,a master LDK is made, and an image used for booting is stored in thisLDK (in FIG. 7, LDK0 is used as the master LDK). Thereafter, an LDK usedfor a client is made, and all of the contents of the master LDK0 arecopied onto this client LDK. This copy operation have to be carried outby the number of client LDKs (in FIG. 7, LDK1, LDK2, and LDK3 are madefor three clients, respectively, and the copy operation from the LDK0 toeach LDK is carried out). Thereafter, an access management table is setto permit access from each client. The reason why copying from themaster LDK0 onto the client LDK is required is to deal with a case inwhich setting change or log-writing is performed from a client uponbooting. A difference created by changing is stored in each LDK (in FIG.7, the difference is shown as TempA, TempB, and TempC).

A second problem is that the capacity efficiency of the entire storageis low. The client LDKs are made by copying from the master LDK, and thecontents stored in one of the client LDKs are the same as each of theother client LDKs if the change difference is excluded from the contentsof each client LDK. Normally, the change difference is small withrespect to the image used for booting, and hence a plurality of clientLDKs in which each client LDK contains almost the same contents as theother client LDKs is provided in the storage. This problem is worsenedin proportion to the number of clients to be connected.

A third problem is that the use efficiency of the cache memory in thestorage is low. The cache memory is used for each LDK when a pluralityof clients make reading requests, respectively. Likewise, in this case,the LDKs have almost the same contents, respectively, and hence wastefuloperations are performed in the same way as in the second problem.

Without being limited to the network boot system, these three problemsalso exist in a network storage system, such as a book reading/browsingsystem (which has a function to send book data from a master LDK to aclient and to allow the client to individually attach a bookmarker tothe book data), in which each client is directly connected to a storageso as to exchange information with each other and in which the clientsshare the information with each other.

Therefore, in this description, the term “network storage system” ishereinafter used as a fairly wide concept including the “network bootsystem.”

SUMMARY OF THE INVENTION

It is an exemplary object of the present invention to provide a networkstorage system capable of reducing the amount of copies in the networkstorage system, heightening storage efficiency, increasing access speed,and improving comfortability, convenience, and economy, and to provide astorage management method, and a control program.

To achieve the exemplary object, a network storage system according toan exemplary aspect of the present invention is formed so that aplurality of client terminals and a storage device that stores data readand written by the client terminals are directly connected to each othervia a network. In this network storage system, the storage deviceincludes an MV logical disk (MV: master volume) that stores read-onlyshared data sharable among the client terminals, a BV logical disk (BV:boot volume) from which or onto which specific data correlated with eachclient terminal is read and written, and a control unit which controlsreading and writing between the logical disks and the client terminals.The control unit determines whether a piece of read command data about aread command issued from a client is present in the BV logical diskcorrelated with each client terminal, and, if the read command data isnot present in the BV logical disk, the control unit reads the readcommand data from the MV logical disk, and performs virtual-memorycontrol to send the read command data read therefrom to the client.

In the above example, the present invention is embodied as a networkstorage system serving as hardware. However, the present invention maybe embodied as a method or a program serving as software.

A storage management method according to another exemplary aspect of thepresent invention is a method of managing a storage device connected toa plurality of client terminals via a network to include an MV logicaldisk (MV: master volume) that stores read-only shared data and a BVlogical disk (BV: boot volume) from which or onto which data specific toeach client terminal is read and written, and the method including stepsof: referring to an LDK management table of the BV logical diskcorrelated with the client terminal; determining whether a logicaladdress is present in a conversion map column of the LDK managementtable; acquiring a physical address from the conversion map andperforming a reading or writing operation if the logical address ispresent in the conversion map column of the LDK management table; andacquiring an MV number from a reference logical disk number column ofthe LDK management table and performing a reading or writing operationfrom the MV logical disk if the logical address is not present in theconversion map column of the LDK management table.

A storage control program product according to still another exemplaryaspect of the present invention is a program product for controlling astorage device connected to a plurality of client terminals via anetwork to include an MV logical disk (MV: master volume) that storesread-only shared data and a BV logical disk (BV: boot volume) from whichor onto which data specific to each client terminal is read and written.A storage control program stored in the storage control program productcauses a computer to execute a function to refer to an LDK managementtable of the BV logical disk correlated with each client terminal; afunction to determine whether a logical address is present in aconversion map column of the LDK management table or not; a function toacquire a physical address from the conversion map so as to perform areading or writing operation if the logical address is present in theconversion map column of the LDK management table; and a function toacquire an MV number from a reference logical disk number column of theLDK management table and then read data thereabout from the MV logicaldisk if the logical address is not present in the conversion map columnof the LDK management table.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structure of a network storagesystem according to an exemplary embodiment of the invention;

FIG. 2 is a schematic view of an access management table according tothis exemplary embodiment;

FIG. 3 is a schematic view of an LDK management table according to thisexemplary embodiment;

FIG. 4 is a flowchart of the operation of a storage device with respectto a read request issued from a client;

FIG. 5 is a flowchart of the operation of the storage device withrespect to a write request issued from a client;

FIG. 6 is a block diagram of a network boot system via a server in arelated technology; and

FIG. 7 is a block diagram of a network boot system in which a server hasbeen removed in a related technology.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Next, a network storage system that is an exemplary embodiment of theinvention will be described with reference to the attached drawings.

As shown in FIG. 1, a client 1 (generally, n (integer) clients may beprovided although three clients, i.e., client A, client B, and client Care provided for descriptive convenience in FIG. 1), which serves as aclient terminal, is directly connected to a storage device 4 via anetwork 2. The client 1 and the storage device 4 can directly read andwrite data by use of the iSCSI protocol.

The storage device 4 is made up of a host interface 5 realized by aninterface control LSI that performs processing to a read request and awrite request from the client 1, a high-speed accessible cache memory 6that temporarily stores the data of a logical disk 7, the logical disk(LDK) 7 logically formed on one or more physical disks, and a controlunit 8 that controls these elements.

Here, prior to giving a detailed description, the main points of thepresent invention will be described.

In the network storage system according to this exemplary embodiment, aplurality of client terminals 1 and the storage device 4 that storesdata read and written by these client terminals 1 are directly connectedto each other via the network 2.

The storage device 4 includes an MV logical disk (MV: master volume)that stores read-only shared data that is sharable among the clientterminals 1, a BV logical disk (BV: boot volume) from which or ontowhich specific data correlated with each client terminal 1 is read orwritten, and the control unit 8 that controls reading and writingperformed between the logical disk 7 and each client terminal 1.

The control unit 8 determines whether data about a read command emittedfrom the client 1 is present or not in the BV logical disk 7 correlatedwith the client 1. If the read command data is not present in the BVlogical disk 7, the control unit 8 reads the read command data from theMV logical disk 7, and performs virtual-memory-like control to send thedata read therefrom to the client.

Therefore, in the control unit 8, data in which two kinds of data, i.e.,read-only shared data, which is enormous in quantity, and specific datadepending on each client are aggregated together is stored and managedin such a manner as to be separated into the MV logical disk 7 and theBV logical disk 7, and these two kinds of data are read and written whennecessary. Since this control unit 8 is provided, it becomes possible toshorten setup time resulting from a reduction in wasteful copy operationperformed before the start of operation between the logical disks 7, andbecomes possible to heighten disk use efficiency.

Additionally, in this exemplary embodiment, the control unit 8 isprovided with an access management table 81 that correlates each client1 with the BV logical disk 7. Additionally, an LDK management table 82is provided in parallel with the access management table 81. The LDKmanagement table 82 issues a command to refer to the MV logical disk 7if a read command data is not present in the BV logical disk 7. Thecontrol unit 8 reads the read command data while referring to the accessmanagement table 81 and the LDK management table 82.

The control unit 8 can easily control MV data and BV data, which aremanaged in a manner divided into two kinds of data, by means of theaccess management table 81 and the LDK management table 82, so that itbecomes possible to provide a system improved in efficiency and economyby restraining the use of a disk capacity and the wasteful operation ofcopying and storing a large amount of MV data by the number of clients1, which is a problem in related technical fields.

The LDK management table 82 has a column used to manage the attribute ofthe logical disk and a conversion map column used to manage the addressmapping of write data, in addition to a reference logical disk numbercolumn used to issue a command to refer to the MV logical disk.

Additionally, the cache memory 6 that temporarily stores read/write datasent from the logical disk 7 is also provided parallel in the controlunit 8. The control unit 8 first determines whether read-command data ispresent or not on the cache memory 6 with respect to a read commandemitted from the client 1 by use of the data holding function of thecache memory 6.

As described above, since data is divided into two kinds of data and isstored in the logical disk 7, it is possible to provide a system inwhich the hit rate of the cache memory can be expected to be heightenedwhen shared data is read, in which the speed of the boot operation canbe increased, and in which convenience and comfortability can beimproved in addition to improvements in efficiency and economy mentionedabove.

Additionally, the Internet using the iSCSI protocol, or LAN, or SANusing the fiber channel protocol can be used as the network 2.

Next, the structure and operation of the storage device will bedescribed in detail.

The storage device 4 is made up of the host interface 5 realized by aninterface control LSI that performs processing to a read request and awrite request from the client 1, the high-speed accessible cache memory6 that temporarily stores the data of the logical disk 7, the logicaldisk (LDK) 7 logically formed on one or more physical disks, and thecontrol unit 8 that controls these elements.

The control unit 8 includes an MPU (not shown) and a local memoryrealized by a semiconductor memory, such as RAM or ROM. The control unit8 is operated by firmware stored in the local memory. The accessmanagement table 81 and the LDK management table 82 are disposed on thelocal memory. The access management table 81 manages the correlation ofany one of the logical disks (LDK) 7 with the client 1. The LDKmanagement table 82 manages the volume attribute of the logical disk(LDK) 7.

As shown in FIG. 2, the access management table 81 consists of clientidentifiers each of which has each individual value for each client toidentify each client and logical disk (LDK) numbers each of which isallocated to each client. In this exemplary embodiment, Initiator Nameseach of which has a value specific to each Initiator in the iSCSIprotocol are set as the client identifiers. Herein, Initiator Name ofclient A is set as Initiator A, Initiator Name of client B is set asInitiator B, and Initiator Name of client C is set as Initiator C.

As shown in FIG. 3, the LDK management table 82 consists of logical disk(LDK) numbers, volume attributes, reference logical disk numbers, andconversion maps. The volume attribute shows the attribute of an LDKindicated by a logical disk number. An LDK which is to be a client LDKis managed as a boot volume (BV), an LDK which is to be a master LDK ismanaged as a master volume (MV), and an LDK which is to be other thanthe client and master LDKs is managed as a normal volume (NV).

The reference logical disk number and the conversion map becomeeffective only when the volume attribute is a BV. The reference logicaldisk number shows the correlation of this BV with any one of the MVs.The conversion map is a table used to convert a logical addressindicated by the client 1 into a physical address, and is renewed when awrite request into the BV is issued from the client 1.

For example, operations to form the access management table 81, the LDKmanagement table 82, and the LDKs can be performed from a maintenanceterminal 3 connected to the storage device 4.

Next, the operation of the network storage system according to thisexemplary embodiment will be described with reference to the attacheddrawings. FIG. 4 is a flowchart followed when a data read request isissued to the storage device 4 from the client 1.

The control unit 8 of the storage device 4 receives a read request fromthe client 1, and then examines the logical disk number from the accessmanagement table 81 based on the client identifier (Initiator Name) ofthe client 1 (step S601, reference step, reference processing).Thereafter, the control unit 8 examines the volume attribute from theLDK management table 82 based on the logical disk number (step S602). Ifthe volume attribute is any one of the volumes except the BV here,normal read processing is performed (step S611).

If the volume attribute is the BV, it is examined whether or not thelogical address of the read request issued from the client 1 isregistered in the conversion map of the LDK management table 82 (stepS604, determining step, determination processing). If the logicaladdress is registered in the conversion map, a corresponding physicaladdress is taken from the conversion map (step S605). Thereafter, dataindicated by the physical address is read from the BV, and istransmitted to the client 1 (step S606, reading step, readingprocessing).

If the logical address is not registered in the conversion map, the MVis examined from the reference logical disk number of the LDK managementtable 82 (step S608). Thereafter, it is examined whether or notcorresponding data is present in the cache memory 6 (step S609). If thecorresponding data is present in the cache memory 6, this data istransmitted to the client 1 (step S607). If the corresponding data isnot present therein, the data is read into the cache memory 6 from theMV (step S610, MV reading step, MV reading processing), and istransmitted to the client 1 (step S607).

Through this operation, data can be read from the MV via the BV, and aresponse can be made to a client when the client issues a read requestto the storage upon booting. Additionally, data read from the MV once atthe reading step is unfolded on the cache memory 6, and hence, when aread request is issued from another client, a response can be expectedto be made to the client at higher speed than data-reading from the MV.As a result, the use efficiency of the cache memory is improved, and theboot time of the client can be quickened.

FIG. 5 is a flowchart followed when a write request to write data intothe storage device 4 is issued from the client 1. When a write requestis received from the client 1, the logical disk number is examined fromthe access management table 81 based on Initiator Name of the client 1(step S701). Thereafter, the volume attribute is examined from the LDKmanagement table 82 based on the resulting logical disk number (stepS702, referring step, referring processing). If the volume attribute isany one of the volumes except the BV here, normal write processing isperformed (step S708).

If the volume attribute is the BV, it is examined whether or not thelogical address of the write request issued from the client 1 isregistered in the conversion map of the LDK management table 82 (stepS704, determining step, determination processing). If the logicaladdress is registered in the conversion map, the physical address istaken from the conversion map (step S705), and data is written onto theaddress indicated by the physical address (step S706, writing step,write processing).

If the logical address is not registered in the conversion map, anunused physical address is calculated, and the conversion map is renewed(step S707). Thereafter, data transmitted from the client 1 is writteninto the calculated physical address (step S706, new writing step, newwrite processing).

As described above, write data can be registered in the conversion map,and can be stored in the BV when a write request is issued afterbooting. Therefore, the amount of data in the BV can be reduced, and thecapacity efficiency of the entire storage can be heightened.

The above-mentioned various steps may be formed as a program executed bythe computer.

In this exemplary embodiment, as an exemplary advantage according to theinvention, an MV LDK can be used from a client LDK at once, and theoperation of copying data from an MV before starting the operation ofthe client LDK becomes unnecessary. Therefore, the setup time isshortened, and the storage use efficiency is heightened, and hence theefficiency, economy, and comfortability of the system are improved.

Additionally, in the BV, write data from a client is managed by theconversion map, and, when data other than this write data is read, aresponse to the client is made by referring to data present in the MV.If a physical disk forming the BV is constructed as a virtual RAID atthis time, data to be actually stored can be reduced, and the capacityefficiency of the entire storage can be heightened.

Additionally, data read by a client from the MV is temporarily stored inthe cache memory, and, when a reading operation is performed fromanother client with respect to the same logical address afterwards, datain the cache memory can be expected to be returned to the client. As aresult, the boot time of the second client terminal and the boot time ofclient terminals subsequent to the second one can be expected to beadvantageously shortened.

An MV logical disk can be used from a client BV logical disk at once,and a copy operation performed to copy data onto a BV logical disk froman MV logical disk before the start of operations can be removed. As aresult, the setup time can be shortened, the storage use efficiency canbe heightened, and the efficiency, economy, and comfortability of thesystem can be improved.

Additionally, a boot image in which two kinds of data, i.e., read-onlyshared data, which is enormous in quantity, and specific data dependingon each client are aggregated together is stored and managed in a mannerseparated into an MV logical disk and a BV logical disk. Furthermore, acontrol unit is provided which has a function to unite these two kindsof data together when necessary. Therefore, a wasteful copy operationperformed between the logical disks before the start of operations canbe removed, and hence the setup time can be shortened, and the disk useefficiency can be heightened.

Additionally, a boot image in which two kinds of data, i.e., read-onlyshared data, which is enormous in quantity, and specific data dependingon each client are aggregated together is stored and managed in aseparated manner, and an access management table and an LDK managementtable are provided both of which have a function to unite these twokinds of data together when necessary. Therefore, the control unit caneasily control MV data and BV data that are managed in a mannerseparated into two kinds. Therefore, the wasteful operation of copyingand storing a large amount of MV data by the number of clients can berestrained, and the use of the disk capacity can be restrained. As aresult, efficiency and economy can be improved.

Additionally, in the BV logical disk, write data transmitted from theclient is managed by the conversion map, and, when data other than thewrite data is read, a response is made to the client while referring todata stored in the MV logical disk. If a physical disk forming the BVlogical disk is made as virtual RAID at this time, data to be actuallystored can be reduced, and the capacity efficiency of the entire storagecan be heightened.

Additionally, data read by a client from the MV logical disk istemporarily stored in the cache memory, and, when the data stored in thecache memory is read by another client afterwards, the data can beexpected to be returned to the client. As a result, for example, theboot time of a client can be also expected to be advantageouslyshortened.

Next, another exemplary embodiment of the present invention will bedescribed. In this exemplary embodiment, the following structure may beformed. In detail, the control unit is provided with an accessmanagement table that correlates each client with the BV logical disk.An LDK management table, which issues a command to refer to the MVlogical disk when the read command data is not present in the BV logicaldisk, is provided in parallel to the access management table. Thecontrol unit reads the read command data while referring to the accessmanagement table and the LDK management table.

Therefore, the control unit can easily control MV data and BV datamanaged in a manner divided into two kinds by using the accessmanagement table and the LDK management table. As a result, a systemimproved in efficiency and economy can be provided by restraining thewasteful operation of copying and storing a large amount of MV data bythe number of clients, which is a problem, and by restraining the use ofthe disk capacity.

Additionally, the LDK management table may be provided with a columnused to manage the attribute of the logical disk and a conversion mapcolumn used to manage the address mapping of write data, in addition toa reference logical disk number column used to give a command to referto the MV logical disk. As a result, efficiency in the data managementof the control unit can be improved.

Additionally, a cache memory that temporarily stores read/write datafrom the logical disk may be provided parallel in the control unit, andit may be first determined whether data about a read command emittedfrom a client is present in the cache memory by use of the data holdingfunction of the cache memory.

As a result, a system can be provided which is capable of heighteningthe hit rate of the cache memory during reading by dividing the datainto two kinds of data, is capable of quickening the boot operation, andis capable of improving convenience and comfortability in addition toefficiency and economy mentioned above.

Additionally, the network may be the Internet or LAN that uses the iSCSIprotocol, or may be SAN that uses the fiber channel protocol.

Additionally, the storage device may be provided with a cache memorythat temporarily stores read data from the MV logical disk, and aprocess for acquiring and reading an MV number from the reference disknumber may include a step of first referring to the cache memory andreading an MV from the MV logical disk if the MV is not present in thecache memory.

As a result, a storage management method can be provided which iscapable of heightening the hit rate of the cache memory during readingby dividing the data into two kinds of data, is capable of quickeningthe boot operation, and is capable of improving convenience andcomfortability in addition to efficiency and economy mentioned above.

Without being limited to the iSCSI protocol, the present invention canbe likewise embodied in a storage system, such as that of the fiberchannel protocol, in which the client 1 and the storage device 4 can bedirectly connected to each other without using a server. For example,Node Name, instead of Initiator Name, can be used as an initiatoridentifier in the fiber channel protocol at that time.

Additionally, the present invention can be embodied not only in thenetwork boot system but also in a system, such as a “bookreading/browsing system,” in which MV data is shared between clients,and a part of the data is renewed by each client. In this “bookreading/browsing system,” each client can directly download and browsebook data from the storage device and can set a bookmarker to the bookdata. In this case, book data is registered in an MV, and a client LDKis set as a BV. Accordingly, a book-data response is made from the MV tothe client when the book data is read, and bookmarker data is stored inthe BV by the conversion map when bookmarker data is written. Therefore,the storage can be efficiently used, and the entire system having ahigh-speed responsiveness can be expected to be improved in convenience,comfortability, and economy.

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, the invention is not limitedto these embodiments. It will be understood by those of ordinary skillin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asdefined by the claims.

1. A network storage system in which a plurality of client terminals anda storage device that stores data read and written by the clientterminals are directly connected to each other via a network, thestorage device including: an MV logical disk (MV: master volume)forstoring read-only shared data sharable among the client terminals; a BVlogical disk (BV: boot volume) from which or onto which specific datacorrelated with each client terminal is read and written; and a controlunit which controls reading and writing between the logical disks andthe client terminals; wherein the control unit determines whether apiece of read command data about a read command issued from a client ispresent in the BV logical disk correlated with each client terminal,and, if the read command data is not present in the BV logical disk,reads the read command data from the MV logical disk, and performsvirtual-memory control to send the read command data read therefrom tothe client.
 2. The network storage system of claim 1, wherein thecontrol unit includes: an access management table for correlating eachclient terminal with the BV logical disk; and an LDK management tableprovided in parallel with the access management table, for emitting acommand to refer to the MV logical disk if the read command data is notpresent in the BV logical disk; and wherein the control unit reads theread command data while referring to the access management table and theLDK management table.
 3. The network storage system of claim 2, whereinthe LDK management table includes a column used to manage an attributeof the logical disk and a conversion map column used to manage addressmapping of write data, in addition to a reference logical disk numbercolumn used to refer to the MV logical disk.
 4. The network storagesystem of claim 1, further comprising a cache memory provided in thecontrol unit, for temporarily storing read/write data from the logicaldisk, wherein when a read command is emitted from a client, the controlunit first determines whether data about the read command is present inthe cache memory by use of a data holding function of the cache memory.5. The network storage system of claim 1, wherein the network is theInternet or LAN that uses an iSCSI protocol, or is SAN that uses a fiberchannel protocol.
 6. A network storage system in which a plurality ofclient terminals and a storage device that stores data read and writtenby the client terminals are directly connected to each other via anetwork, the storage device including: an MV logical disk (MV: mastervolume)for storing read-only shared data sharable among the clientterminals; a BV logical disk (BV: boot volume) from which or onto whichspecific data correlated with each client terminal is read and written;and a control means for controlling reading and writing between thelogical disks and the client terminals; wherein the control meansdetermines whether a piece of read command data about a read commandissued from a client is present in the BV logical disk correlated witheach client terminal, and, if the read command data is not present inthe BV logical disk, reads the read command data from the MV logicaldisk, and performs virtual-memory control to send the read command dataread therefrom to the client.
 7. A storage management method formanaging a storage device connected to a plurality of client terminalsvia a network to include an MV logical disk (MV: master volume) thatstores read-only shared data and a BV logical disk (BV: boot volume)from which or onto which data specific to each client terminal is readand written, the storage management method comprising: referring to anLDK management table of the BV logical disk correlated with the clientterminal; determining whether a logical address is present in aconversion map column of the LDK management table; acquiring a physicaladdress from the conversion map and performing a reading or writingoperation if the logical address is present in the conversion map columnof the LDK management table; and acquiring an MV number from a referencelogical disk number column of the LDK management table and performing areading or writing operation from the MV logical disk if the logicaladdress is not present in the conversion map column of the LDKmanagement table.
 8. The storage management method of claim 7, furthercomprising temporarily storing read data read from the MV logical diskin the cache memory, thereafter first referring to the cache memory whenthe MV number is acquired and read from the reference logical disknumber column, and reading the MV number from the MV logical disk if anMV is not present therein.
 9. A storage control program product forcontrolling a storage device connected to a plurality of clientterminals via a network to include an MV logical disk (MV: mastervolume) that stores read-only shared data and a BV logical disk (BV:boot volume) from which or onto which data specific to each clientterminal is read and written, the storage control program productstoring a storage control program causing a computer to execute: afunction to refer to an LDK management table of the BV logical diskcorrelated with each client terminal; a function to determine whether alogical address is present in a conversion map column of the LDKmanagement table; a function to acquire a physical address from theconversion map so as to perform a reading or writing operation if thelogical address is present in the conversion map column of the LDKmanagement table; and a function to acquire an MV number from areference logical disk number column of the LDK management table andthen read data thereabout from the MV logical disk if the logicaladdress is not present in the conversion map column of the LDKmanagement table.